The present invention relates to radiation detection, and more specifically to single polarity charge carrier sensing in ionization detectors.
Radiation detectors using simple planar electrodes and based on ionization measurements often suffer from poor collection of charge carriers of certain polarity types. For example, positive charge carriers (holes) may migrate through the detector medium at a much slower rate than negative charge carriers (electrons). As a result, such detectors produce signals that vary in amplitude depending on the location within the detector at which incident radiation interacts with the detector medium. Such detectors include semiconductor detectors, liquid ionization detectors, and gas ionization detectors.
In a simple planar electrode ionization detector, full-area electrodes are formed on two opposing faces of the detector medium. A bias voltage applied across the two electrodes provides an electric field to separate and collect the charge carriers that are created by the absorption of radiation in the detector medium. Induced charge signal on one of the electrodes due to the motion of carriers provides a measure of the energy of the radiation. Incomplete charge collection due to carrier trapping or slow carrier transport results in reduced signals, which vary in strength depending on the depth of radiation interaction. This degrades the energy resolution of the detector.
U.S. Pat. No. 5,530,249 describes a method and apparatus to improve the energy resolution of ionization-type radiation detectors suffering from incomplete charge collection. Two interlaced or interdigitated electrodes are used to sense the movement of charge carriers within the detector. The induced charge signals on these electrodes are subtracted to give a net signal that yields substantially improved energy resolution.
Thus, by reconfiguring the charge sensing electrode on a detector into a pair of interdigitated electrodes, the signal response can be modified such that the signal amplitude variation caused by poor carrier transport properties is greatly reduced. The coplanar interdigitated grid detector uses two interdigitated electrodes on the detector for charge sensing. The desired signal response is obtained by subtracting the induced signals on the two grid electrodes. By changing the relative gain of the two signals before subtraction, the detector response can be effectively tuned to match the charge transport properties of the material and thus optimize the spectral response.
While the two-electrode readout interdigitated grid detector is far superior to the full-area electrode detector, there are the problems of more complex and costly electronic circuitry, involving a two channel amplifier system with subtraction circuit, and greater electronic noise. Thus it would be desirable to have a detector which has the advantages of the interdigitated electrode structure, but with simpler electronics.